The present invention relates to an automatic calibration circuit for a monitoring device for an image recording apparatus in which a video signal corresponding to a static image drives a CRT of the monitoring device to display the static image thereon and that image is projected and recorded on a recording medium.
Recently, medical diagnostic apparatuses have come into common use in which static images, such as ultrasonic or X-ray diagnostic images, are converted into video signals, the video signals are applied to drive a CRT to display the images, and the images are photographed from the display surface of the CRT.
The operator of such an apparatus may make initial brightness and contrast settings while looking at a test image appearing on the CRT so that images having optimum gradation and brightness are available for photographic recording. However, even if the initial setting is satisfactory, after a time the displayed images may become unsatisfactory and the developed film unusable for diagnosic purposes because the CRT and associated electronic circuits are subject to drift over time depending on the operating conditions (temperature and humidity), resulting in undesirable brightness and contrast changes.
In order to remedy such shortcomings, the conventional apparatus utilizes the AE (automatic exposure) mechanism of a camera whereby the light at one point on the CRT is measured or the light at several points on the CRT is averaged to correct for such drift. However, the measured brightness of the image may still different from what is desired because of differences in position, background color, etc. Consequently, this correction method is not fully acceptable.